Faraday-effect element

ABSTRACT

A novel Faraday-effect element having a refractive index highest along the axis and decreasing toward the surface is produced by immersing a thin elongated glass body containing an oxide selected from the group consisting of paramagnetic- and diamagnetic-type oxides composed of at least first cations, in a bath of salt including second cations having a smaller ration of the electronic polarizability to the third power of the ion radius than said first cations. The bath is maintained at a temperature to permit said second cations to diffuse into said glass body so that said first cations may be substituted by said second cations.

SEARCH ROOM United btates [111 3,633,992

[72] Inventors Teljlllchldl; 3,083,123 3/1963 Navias 350/!75 UX MotelFumhwnShopYedlnmall 3.320,l l4 5/l967 Schulz..... 350/96 UX at Tokyo;lchbe Kltm; Ken Kola-l, 3,434,774 3/1969 Miller 350/96 both ofKobe-shim! elm OTHER REFERENCES 1 7 2, Miller, Light Propagation inGeneralized Lens-Like gr a "n Media," Bell System Tech. 1., Vol. 44, No.9 (Nov. 1965) pp.

2,0l7- 2,030 [73] Assume Kawakami et al., Propagation Loss in aDistributed Beam [32] Priorities $2 1m Waveguide, Proc. IEEE (Dec. 1965)pp. 2,l48- 2,149 I 33 1 Primary Examiner--- David Schonberg 43 473Assistant Examiner-Paul R. Miller 31 55.

5. 9 4314735 Attorney-Sand, Hopgood and Calimafde [54] FARADAYJFFECTELEMENT ABSTRACT: A novel Faraday-effect element having a refrac-4Clallns,4Dra\vlng Figs. tive index highest along the axis anddecreasing toward the 350" surface is produced by immersing a thinelongated glass body [52] U.S.Cl 350/961 comainin an oxide select fromthe group consisting of cl 1/22 paramagneticand diamagnetic-type oxidescomposed of at 350/96 w least first cations, in a bath of salt includingsecond cations [50] F d o having a smaller ration of the electronicpolarizability to the 96 15 l 96 96 175 SN; 324/96 third power of theion radius than said first cations. The bath is maintained at atemperature to permit said second cations to [56] Rdemm diffuse intosaid glass body so that said first cations may be UNITED STATES PATENTSsubstituted by said second cations. 3,030,852 4/1962 Courtney-Pratt350/l5l X slsaalss'z PATENTED JAN! 1 i972 SHEEI 2 BF 2 2 mm F 1 I00 (c)200 300 Depth Measured from Surface (Microns) 5 4 R u a m mmo n A m M mwmww m WHFOMM T c Y 0 t A u x M M mm mmsxx FARA DAY-EFFECT ELEMENTBACKGROUND OF INVENTION This invention relates to a Faraday-effectelement, and, in particular, such an element for use as an opticalisolator. The Faraday effect is the property of transparent substancesby which the plane of polarization is rotated when the material isplaced in a magnetic field, for light propagated along the magneticfield.

The angle of the rotatory polarization of a Faradayeflect element isgiven by 0=VHL, where H stands for the magnetic.

field intensity in the direction of travel of the light wave, L for theoptical length of the light transmission medium in the magnetic field,and V is a constant. The constant V is called Verdets constant and isdeemed positive when it gives rotatory polarization in the direction ofthe current flowing through the solenoid being provided to generate themagnetic field H. It is well known that the direction of the rotatorypolarization depends on the direction of the magnetic field and not onthe direction of travel of the light wave being subjected to therotatory polarization.

In order to be suited for use in a Faraday-effect element the materialemployed should have a well-stabilized Verdet's constant V of largeabsolute value, and small attenuation constant. In other words, such amaterial should be transparent and not produce any adverse effects uponthe wave surface of the light wave. Materials which meet theserequirements include'some types of glass containing paramagnetic ordiamagnetic materials. Heavy lead silicate glass,terbium-alumina-silicate glass, terbium-metaphosphate glass, andceriumphosphate glass are typical examples. However, in order to obtaina rotatory polarization of 1r/4 radian with such Faraday-effectmaterials, assuming a magnetic field of 1,000 oersteds, the optical pathwithin the material would extend over scores of centimeters. If anauxiliary lens is not sufficient to completely remove the divergence atthe Faraday-effect element, the cross section normal to the axis shouldbe large enough to tolerate the divergence. However, a glass light guideof large cross section is not easy to bend.

OBJECT OF INVENTION Accordingly, it is the object of the presentinvention to provide Faraday-efiect elements of converging capabilitywhich overcome the above-mentioned difficulties.

BRIEF SUMMARY OF INVENTION Briefly, the invention is predicted upon theconcept of providing a fibrous glass element which has paramagnetic ordiamagnetic properties with a reflective index which is highest at theaxis and gradually decreases toward the surface. According to the methodof the invention, such an element may be produced by substituting fromthe surface cations of lower electron polarization/( ion radius) forthose initially present in the glass.

GENERAL DESCRIPTION OF THE INVENTION In a paper published in the BellSystem Technical Journal, July I964 issue, pages 1,469-1 ,479, D.W.Berreman describes a so-called gas lens in which gas Within a pipe isgiven a gradient in the refractive index to avoid the divergence in alight beam travelling therethrough. Also, in a contributed paper by S.Kawakami and J. Nishizawa published in the Proceedings of the IEEE, Dec.1965 issue, pages 2,l482,l49, an application of the theory of the gaslens to the solid-state transparent body to form a fibrous converginglight guide is suggested.

In such a converging light guide, a light beam incident at one endsurface is propagated along the light guide oscillating about its axis,without diverging. Thus, the phase velocity difference and thedivergence of the beam observed at the output end surface of the lightguide are minimized in such a converging light guide. As in the case ofthe gas lens, an optical image projected on the input end surface of thelight guide can be transmitted substantially as is to the output end.Furthermore, as long as the radius of curvature of the guide is greaterthan a specific lower limit, the bending of the light guide will notproduce any adverse efi'ect on the propagation of the light beam withinthe light guide.

More specifically, the transmission of an optical image is made possibleonly when the refractive index symmetrically decreases toward thesurface of the light guide with respect to the axis of the light guide,and particularly when the refractive index n, is given by =m; lnr').where r stands for the radial distance from the axis in cross section ofthe light guide, n. for the refractive index at the axis, and a for apositive constant.

The spot size W of the light beam of the fundamental mode matched with alight guide of the above-mentioned refractive index distribution isgiven by u 2 l I- is where A stands for the light wavelength in freespace. The mode-matched light beam has a constant cross section withinthe light guide. Also, the optical paths taken by a plurality of lightbeams and viewed at a plane normal to the direction of the travelling ofthe light beams are quite regular. The function of the light guiderestraining the light beam from diverging is referred to hereunder asthe converging property.

The refractive index of glass depends principally on its composition.Therefore, a refractive index gradient may be given to a Faraday-effectelement of the fibrous light guide type by giving a suitable gradient incomposition to a glass of Faraday rotation capability so that therefractive index decreases from the axis toward the surface thereof.

It is well known in this technical field that the rare earth elementssuch as cerium, europium, and terbium give paramagnetic property toglass and that lead, thallium and other similar metals make glassdiamagnetic. It has been deemed almost impossible, however, to provide afibrous glass light guide which has not only a gradient in therefractive index, but also paramagnetic or diamagnetic properties.

In general, oxide glass is composed of glass-composing oxides (SiO,, 8,0P 0 etc.) and modifying oxides. Glass having a paramagnetic ordiamagnetic property contains the oxides of paramagnetic or diamagneticproperty as the modifying oxides. We have discovered that if the ratioof each of the oxides contained in the modifying oxides is graduallychanged toward surface while the molecular ratios of the glass-composingoxide to the modifying oxides are kept unchanged, a glass body isobtained whose refractive index changes gradually in the directionnormal to the axis of the light guide. Based on this discovery, it hasbecome possible to provide the Faradayeffect elements of paramagnetic ordiamagnetic property whose refractive index decreases from the axistoward the surface of the light guide.

In general, the refractive index of a substance is closely related tomolecular refraction and molecular volume inherent to the substance.More specifically, the greater the molecular refraction is, or thesmaller the molecular volume is, the greater is the refractive index. Onthe other hand, the molecular refraction is proportional to thepolarizability of the substance. In general also, the molecularrefraction of glass is approximated by the summation of molecularrefractions of individual ions. Therefore, the qualitative effect of theexistence of those ions on the refractive index of glass can bedetermined by comparing, within the glass, values of electronicpolarization per unit volume of the related ions, or the valuesexpressed by (Ion radiusy Among the cations capable of composing themodifying oxides, lithium, sodium, potassium, rubidium, cesium, andthallium are typical examples suited for changing the above-mentionedratio. Table 1 shows, as to each of these ions, the ion radius, electronpolarizability, and the above mentioned ratio, electron polarizability/(ion radius Since each ion has its inherent ratio of the electronpolarizability to (ion radius), the refractive index of the glasscontaining cations composing modifying oxides is larger than that ofglass whose above-mentioned cations are totally or partially substitutedby the cations having the ratio smaller than those of theabove-mentioned oxides.

As will be clearly understood from the foregoing, a Faradayeffectelement of the fibrous light guide type which has the refractive indexdecreasing toward the surface of the light guide within the plane normalto its axis can be obtained by increasing the concentration in glass ofthe modifying-oxidesconstituting cations towards its surface. Thisincrease in the cation concentration results in the decrease, in thedirection from the glass body axis toward its surface, of theconcentration of other cations, for example, thosehaving paramagnetic ordiamagnetic properties which compose the modifying oxides having greaterelectron-polarizability-to-(ion radius) ratio than the cations composingthe modifying oxides.

The modifying-oxide-composing cations contained in glass may be of threeor more kinds. Assuming, for example, that three different kinds ofcations having mutually different electron-polarizability-to-(ionradius) ratios are called A ion, B ion, and C ion, in the order of thevalue of the ratio, a glass body in which the concentration of B ionincreases in the direction from the interior toward the surface whilethose of both ions A and C decrease in the same direction, has arefractive index distribution which gradually decreases toward thesurface if the difference of C and B ions in theelectronpolarizability-to-(ion radius) ratio is considerably smallerthan the corresponding difference between B and A ions, or if thevariation in the concentration of C ions is smaller than any of those ofA and B ions. In other words, the refractive index assumes adistribution within a plane normal to the light guide which is highestat the axis and decreases toward the surface thereof, because theexistence of A and 8 ions cancels the effect of C ions. Assuming furtherthat the A ion is a diamagnetic ion of great electron polarizability asis exemplified by T1, and that B and C ions are similar ions ofrelatively small electron polarizability as exemplified respectively byK" and Na", the Verdets constant V is held to gradually decrease towardthe surface of the Faraday-effect element.

According to the present invention, the Faraday-effect element ismanufactured through the following process. A fibrous glass containingfirst cations capable of composing the modifying oxides is brought intocontact with such salt as includes second cations capable of composingthe modifying oxides of smaller electron-polarizability-to-( ion radius)ratio, so that first cations distributed on the surface region of theglass may be substituted by the second cations in the salt. Tofacilitate the substitution, the salt and glass are heated up to thetemperature at which both the first and second cations are able todiffuse within the glass. As a result of the diffusion of the secondcations into the glass through the boundary surface between the salt andglass. a portion of the first cations which have been contained withinthe glass emerges from the glass through diffusion. This results in thesubstitution of the first cations in the surface region ofthe glass bythe second cations in the salt. The concentration of the second cationsdiffused into the glass is highest at the surface of the glass body anddecreases toward its axis.

Conversely, the concentration of the first cations which existed withinthe glass in the initial stage of processing tends to decrease at thesurface region. It is highest at the axis of the glass body anddecreases toward its surface in proportion to the distance from theaxis. It follows, therefore, that after the ion substitution process therefractive index is lowest at its surface and increases toward its axisin proportion to the depth. The refractive index at the axis wouldvirtually be that of the glass body before being subjected to the ionsubstitution.

The refractive index distribution within the interior of the glass bodyis affected by various conditions. As to an ordinary elongatedcylindrical glass body having a circular cross section, the refractiveindex distribution is dependent upon the dimension and geometry of theglass body before the ion substitution process upon the compositionof'the salt for the ion substitution bath, and upon the temperature andduration of the substitution processing. Since the degree of the iondiffusion within the glass body depends on the distance from the surfacebrought into direct contact with the salt, the refractive index of thecylindrical rod-shaped glass body has, after the ionsubstitution, asymmetrical distribution of the refractive index with respect to theaxis of the body. When viewed at its circular cross section, therefractive index is symmetrically dependent on the radial distance fromthe axis. Selection of the various conditions of the above-mentionedprocessing make it possible to approximate the refractive index gradientto the ideal quadratic one.

The Faraday-effect element manufactured according to this inventionprovides an optical isolator particularly adapted to theultra-high-speed pulse signal transmission, because phase velocitydifference and light beam divergence are completely avoided within theglass body.

The Faraday-effect element according to this invention does not need anyauxiliary lens for light beam convergence, because of theabove-mentioned converging property. Also, the element itself is formedinto a thin glass fiber having a circular cross section of several tensof microns in diameter. This facilitates the miniaturization of thesolenoid coil employed for generating magnetic field to be applied tothe Faraday-effect element, facilitating the miniaturization of theoptical isolator as a whole. Also, the flexibility and the convergingproperty of the fibrous body allow the isolator to be arbitrarily bent.Particularly, when the refractive index gradient is made greater or, inother words, when the above-mentioned constant a is made greater, thespot W can be made smaller, allowing a greater bend in the fibrous body.

The mode-matched light beams made incident upon the light guide at theinput end surface of the fibrous element travels through the elementoscillating about its axis, except for such a light beam as is madeincident perpendicularly to the input end surface at its center, thisbeam travels along the axis. it is assumed here that the magnetic fieldapplied to the Faraday-effect element along its axis is uniform over itscross section normal to the axis. Since the diameter of the element issufficiently small, this assumption is easily justified.

The above-mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent and the inventionitself will best be understood by reference to the following descriptionof the invention taken in conjunction with the accompanying drawings,the description of which follows.

FIGS. 1 and 2, respectively, show the refractive index characteristicsand ion concentration distribution of the Faraday-effect elementmanufactured according to the present invention; and

FIGS. 3 and 4 show the Faraday-effect element of the invention as anoptical isolator.

DETAILED DESCRIPTION OF THE INVENTION Example I A thin glass rod of acircular cross section of 0.5 mm. in diameter containing 14.0 weightpercent of Tl,O, 66.2 weight percent of PhD, and l9.8 weight percent ofSiO, is immersed for 16 hours in a potassium nitrate bath containing 0.5weight percent of thallium nitrate and maintained at 400C. The glass rodis removed from the bath, rinsed and dried. The refractive index N, atits axis and surface is 1.92 and 1.91, respectively. The refractiveindex gradient within the glass rod is as shown in FIG. 1. This curve isapproximated by the above-mentioned expression n,=n lar with a=8.34 cm.

The glass rod is then subjected to the electromicroprobe X- ray analysisto determine the concentration of Pb, TI", and K" ions. The result is asshown in FIG. 2, in which the ordinates show relative values ofconcentration. These graphs clearly show that a portion of TI ions weresubstituted by K ions, and that this substitution has clearcorrespondence to the refractive index gradient shown in FIG. I. TheVerdets constant for a light wave of 0.6328 micron in wavelength showsvery little variation taking the value at 0.085 min./e cm., as shown inFIG. 1.

Two end surfaces of the glass rod are then polished to form a 30 cm.long rod having smooth end surfaces normal to its axis. After disposingthe glass rod along the axis of a solenoid, the solenoid is energized toapply a magnetic field to the rod. With suitable adjustment of theenergizing current, rotatory polarization of 11/4 radian is attained asto the mode-matched output light beam of a He-Ne laser. The error in theangle of the rotatory polarization and the loss caused by the light beamdispersion are virtually negligible. No difference in the phase velocityis observed in the light wave which has passed through the glass rod.

Example 2 A thin glass rod of a circular cross section of 0.2 mm. indiameter containing 20.0 weight percent of Tl,0, l2 weight percent ofNa,O, 20 weight percent of PbO, and l 1.8 weight percent of SiO,, isimmersed for 12 hours in a potassium nitrate bath maintained at 450 C.The glass rod is then removed from the bath, rinsed and dried. Therefractive index n at its axis and surface is 1.60 and 1.57,respectively. The gradient in the refractive index in the rod isapproximated by 'li' rtiljfl'ilefilflf 8. 5. .9 (m;.)- Il1... .n9=m ofTl and Na ions decreases toward the surface of the rod, while theconcentration of K ions increases toward the surface, it is observed.The experimental results quite similar to the rod of example I wereobtained as to the 60 cm. long sample of the example 2.

Example 3 A glass rod of circular cross section of 0.2 mm. in diametercontaining 10 weight percent of C5 0, l6 weight percent of Na,0, 10weight percent of PbO, and 64 weight percent of SiO, is kept immersedfor 24 hours in a potassium nitrate bath maintained at 400 C. The glassrod is then removed from the bath, rinsed. and dried. The refractiveindex n,, at its axis and surface is 1.52! and 1.507, respectively. Therefractive index distribution is well approximated by the expressionn=n,,( l-ar), with a=92(cm. It is observed after the above-mentionedprocess that the concentration of P ions is kept constant, while that ofCs* ions decreases and that of Na ions increases in the direction fromthe axis of the rod toward its surface. The experimental resultsobtained with regard to a I00 cm. long sample of example 3 proved to bequite similar to those of example I.

In FIG. 3, which schematically shows the Faraday-effect elementmanufactured through the above-described process. a solenoid 12 18disposed surrounding the Faraday-effect rod 1 I.

To the input and the output end surfaces of the rod II is attached apair of polarizer plates 13 and 14. The polarizer l4 at the output endserves as the so-called analyzer. The light beam made incident upon therod 11 at its polarizer I3 in the direction shown by an arrow, islinearly polarized by the first polarizer 13 and subjected to therotatory polarization while traveling through the rod 11, to which themagnetic field is applied by the solenoid 12. Adjusting the magneticfield intensity to a favorable value through control of the currentflowing through the solenoid, the rotatory polarization of 1r/4 radianis easily attained.

In the embodiment of FIG. 4, the Faraday-effect element 21 is formedinto a coil. Along the axis of the coil-shaped element 21, a conductoris disposed to apply circular magnetic field to the coil 21. At theinput and output ends of the element 21, a pair of polarizers 23 and 24are attached, corresponding to polarizers l3 and 14 ofthe embodiment ofFIG. 3.

While the principles of the invention have been described in connectionwith specific apparatus, it is to be clearly understood that thisdescription is made only by way of example and not as a limitation tothe scope of the invention.

What is claimed is:

l. A Faraday-efiect element made of fibrous cylindrical glass disposedin a magnetic field substantially parallel to the cylindrical glassaxis, said glass containing at the said axis 2 to 65 percent by weightof T1 0, less than 75 percent by weight of PbO, and 15 to 70 percent byweight of SiO,, wherein the concentration of Tl,0 decreases continuouslytoward the surface of said glass, whereby the refractive index n,- ofsaid glass decreases substantially according to the relation n,=n.,( larwhere n, is the refractive index at said axis. r is the radial distancefrom said axis, and a is a positive constant.

2. A Faraday-effect element as claimed in claim I, wherein I the two endsurfacesgf said element are polished into opti-

1. A Faraday-effect element made of fibrous cylindrical glass disposedin a magnetic field substantially parallel to the cylindrical glassaxis, said glass containing at the said axis 2 to 65 percent by weightof Tl2O, less than 75 percent by weight of PbO, and 15 to 70 percent byweight of SiO2, wherein the concentration of Tl2O decreases continuouslytoward the surface of said glass, whereby the refractive index nr ofsaid glass decreases substantially according to the relation nr no(1-ar2) where no is the refractive index at said axis, r is the radialdistance from said axis, and a is a positive constant.
 2. AFaraday-effect element as claimed in claim 1, wherein the two endsurfaces of said element are polished into optically smooth surfacesnormal to said axis.
 3. A Faraday-effect element as claimed in claim 1,wherein said element is rectilinear and disposed in parallel with themagnetic field generated by a solenoid along whose axis said element isdisposed.
 4. A Faraday-effect element as claimed in claim 1, whereinsaid element is formed into a helix and disposed in parallel with themagnetic field generated by a rectilinear conductor laid along the axisof said helix.